Fizeau plate for use in multiple beam interferometers



Waited States atent m1 amas Inventors Lawrence F. Herte Palo Alto; Frank R. Kloss, San Francisco, California Appl. No. 626,155 Filed March 27, 1967 Patented Nov. 24, 1970 Assignee Varian Associates Palo Alto, California a corporation of California FIZEAU PLATE FOR USE IN MULTIPLE BEAM INTERFEROMETERS 2 Claims, 4 Drawing Figs.

US. Cl 356/109, 350/12 int. Cl (i02b 2l/06; G0 I b 9/02 Field of Search 350/1 2,

334. I57; 356/lO7-l09, 120

[56] References Cited UNITED STATES PATENTS 3,442,594 5/1969 Minard et al. 350/8IX 3,352,198 11/1967 Klute 350/l2X 3,046,837 7/1962 Barabas et a]. 350/! 3X FOREIGN PATENTS 631,25l l0/l949 Great Britain 350/14 OTHER REFERENCES Born, et al. PRINCIPLES OF OPTICS, The MacMillan Co., NY. l964,pp 289- 291 Primary Examiner-Paul R. Gilliam Attorneys-William]. Nolan and Leon F. Herbert ABSTRACT: A Fizeau plate for use in optical interferometers. A portion of the Fizeau plate has a reflective coating while the remainder is transparent to permit direct viewing for identification of an adjacent region of the specimen whose surface variations are being observed.

Patented Nov. 24,1910 3,542,413

62 6| 59 4 INVENTORS LAWRENICE [mama FRANK n. moss BY ATTORNEY BACKGROUND OF THE INVENTION The present invention relates generally to multiple beam iiiterferometers, and more particularly to an improved Fizeau plate for use in such interferometers to permit simultaneously viewing of a portion of the specimen surface and interference pattern of an adjacent portion.

In known multiple beam interferometers, a specimen to be analyzed for microscopic variations in surface contour on the order of to 20,000 Angstroms is brought into contact with an optical flat or Fizeau plate, with a predetermined air wedge angle between them. A source of monochromatic 'light irradiates the specimen and Fizcau plate in a direction normal to the surface of the specimen. A microscope is used to provide a magnified image of the interference lines which appear in the air wedge. Measurements of shifts in line spacing pro vide an accurate indication of minute surface variations.

Many applications ofthe optical interferometer require that the portion of the specimen which is being viewed be readily and accurately identifiable. For example, in evaluation of the coating thicknesses of various portions of'a microelectronic circuit, precise determination of the region of the specimen being observed is essential. Prior art interferometers have made no provision for identifying the portion ofthc specimen being observed other than interpretation of the interferogram itself or time-consuming removalof the Fizeau plate for direct optical observation of the specimen. While interpretation of the interferogram is. satisfactory 'for the identification of several different coatings when these coatings differ in thickness by an order of magnitude or more, the method becomes very uncertain when coating thicknesses differ by only a small amount.

SUMMARY OF THE INVENTION According to the present invention the necessary identification is accomplished by direct optical observation of a portion of the specimen within the field of view ofthc microscope lens system while at the same time the interference fringe pattern ofan adjacent portion ofthe specimen within the field of view is observed.

Accordingly, it isan object of the present invention to provide a multiple beam interferometer having means for con venient direct optical viewing ofthe specimen.

It is a further object to provide a multiple beam interferometer in which a portion of the specimen can be observed simultaneously with the interferencev fringe pattern of an ad jacent portion.

The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of one specific" embodiment thereof, especially when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING FIG. I is an illustration of a conventional reflection type multiple beam interferometer;

FIG. 2 is a schematic diagram of the optical elements eniploycd in a multiple beam interferometer according to the present invention;

FIG. 3 is a side view of an optical tlat or Fizeau plate according to the present invention; I

FIG. 4 is a view taken along line 4-4 of FIG. 3, showing the lower surface ofthe Fizeauplate which contacts the specimen.

DESCRIPTION THE PREFERRED EMBODIMENT Referring to FIGS. I and 2, a multiple beam reflective type interferometer 11 for producing Fizeau fringe patterns which are representative of the contour of the surface ofa specimen 23 residing on specimen plate I2 comprises a monochromatic light source 13, such as sodium vapor lamp, mounted at right angles to the reflective viewing path. An optical illuminating system 14 is arranged to receive andproject an image of source I3 at a selected image point 16'. Such an illuminating system 14 will generally comprise a condensing lens I? mounted between an adjustable field of illumination delining iris l8 and an adjustable field-intensity-defining iris 19. A half silvered mirror 20 is positioned to receive the light from the optical system I4 and project it towards the Fizeau plate 21 and specimen plate I2 ofinterferometer I l.

i In multiple beam interferometers, a parallel beam of light is employed to form thc Fizeau fringe patterns. To generate the required parallel beam of light, an objective lens 22 is mounted so that image point l6 lies at its back focus. The light at image point 16 is rendered parallel by the objective lens 22 to fall on Fizeau plate 2I and specimen 23. Plate 2] and specimen 23 must be positioned just beyond the focal point of the objective lens 22 in order that an image of the fringe pat' tern produced by eye piece 1 located on the side of mirror 20 opposite objective lens 22. Objective lens 22 and eye piece I together form a microscope lens system for viewing the Fizenu plate 21 and specimen 23.

The Fizeau fringe pattern is generated by positioning the plates I2 and 21 to define an air wedge of very small angle between Fizeau plate 21 and specimen 23. The size of the air wedge angle defined by-the plates is adjusted in accordance with the size of the surface variations to be encountered by turning screws 46 and 56 which bear against a Fizeau plate carrier (not shown) within housing 3I. Specimen plate holder 57 is spring loaded in an upward direction to support specimen 23 in contact with Fizeau plate 21.

Turning now to FIGS. land 4, the [Fizeau plate or optical flat 21 of the present invention is shown. Fizeau plate 21 may be made of optical glass or quartz. Upper surface 58 and lower surface 59 are ground and polished so that each is optically flat and transparent to a high degree. During this operation, surfaces 58 and 59 are brought into parallelism. Cylindrical edge 60 is ground and may he opaque or translucent.

I As seen in FIG. 4 a central circular portion 61 of lower surface 59 is coated with a substance which is partially reflective at the wavelength of'light used for observation. For example, the coated portion til-might reflect 94 percent of the light incident upon the surface from sodium vapor lamp I3 and trans mit the remaining 6 percent. The surrounding annular portion 62 is not coated and remains optically transparent.

As a result of this coating, interference fringe lines will be formed under central portion 61 in the air wedge between I Fizeau plate 21' and may be observed at eye piece I by moving objective lens 22 to bring these lines into focus.

Fizeau plate 21 is typically dimensioned the same size as or stnaller than the field of view of the microscope lens system and may be, for example, (,1. l 50 inches in diameter. Moreover, the wedge angles typically used are so small that the inter ference fringe lines formed at the lower surface 59 under the central portion 61am very nearly in the same plane as the surface of specimen 23. Hence, when objective lens 22 is moved to bring the fringe lines into focus at eye piece I, the microscope lens system is also, to a good approximation, in focus on the surface of specimen 23. 7

Thus the image seen through eye piece I includes not only a circular central portion displaying the interference fringe lines but also a surrounding annular portion displaying an image of the surface of the specimen.

In this manner the operator can rapidly and conveniently identify the region of the specimen being observed.

We claim:

1. An interferometer for analyzing a specimen by inspection of interference fringe patterns, said interferometer comprising an optical plat having a partially reflective coating on one surface thereof, means for positioning said optical flat adjacent a specimen to be inspected to define an air wedge therebetwcen, a microscope lens system defining a field of view for magnified observation of said flat and said specimen, and a source oflight positioned to irradiate said plate and said specimen iii a direction parallel to the optical axis of said lens system for producing interference fringcs between the light reflected from saidcoating and said specimen. said coating covering a portion olsaid one surface smaller in area than the area ol said one surface within said field of view whereby a direct optical image oi a portion of said specimen-can he ohserved with said microscope lens system through the portions Ill 

